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Abstract:

A wafer processing method transfers an optical device layer (ODL) in an
optical device wafer (ODW) to a transfer substrate. The ODL is formed on
the front side of an epitaxy substrate through a buffer layer, and is
partitioned by a plurality of crossing streets to define a plurality of
regions where optical devices are formed. The transfer substrate is
bonded to the front side of the ODL. The transfer substrate and the ODL
cut along the streets. The transfer substrate is attached to a supporting
member, and a laser beam is applied to the epitaxy substrate from the
back side of the epitaxy substrate to the unit of the ODW and the
transfer substrate. The focal point of the laser beam is set in the
buffer layer, thereby decomposing the buffer layer. The epitaxy substrate
is then peeled off from the ODL.

Claims:

1. An optical device wafer processing method for transferring an optical
device layer in an optical device wafer to a transfer substrate, said
optical device layer being formed on the front side of an epitaxy
substrate through a buffer layer, and partitioned by a plurality of
crossing streets to define a plurality of regions where optical devices
are respectively formed, said optical device wafer processing method
comprising: a transfer substrate bonding step of bonding said transfer
substrate to the front side of said optical device layer formed on the
front side of said epitaxy substrate through said buffer layer; a
transfer substrate cutting step of cutting said transfer substrate bonded
to the front side of said optical device layer together with said optical
device layer along said streets; a supporting member attaching step of
attaching said transfer substrate to a supporting member after performing
said transfer substrate cutting step; a peeling laser beam applying step
of applying a laser beam having a transmission wavelength to said epitaxy
substrate from the back side of said epitaxy substrate to a unit of said
optical device wafer and said transfer substrate in a condition where the
focal point of said laser beam is set in said buffer layer, thereby
decomposing said buffer layer; and an epitaxy substrate peeling step of
peeling off said epitaxy substrate from said optical device layer after
performing said peeling laser beam applying step.

2. The optical device wafer processing method according to claim 1,
wherein said transfer substrate cutting step comprises the step of
cutting said transfer substrate along said streets by using a cutting
blade.

3. The optical device wafer processing method according to claim 1,
wherein said transfer substrate cutting step comprises the step of
cutting said transfer substrate along said streets by applying a laser
beam to said transfer substrate along said streets.

Description:

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical device wafer processing
method for transferring an optical device layer in an optical device
wafer to a transfer substrate, the optical device layer being composed of
an n-type semiconductor layer and a p-type semiconductor layer, formed on
the front side of an epitaxy substrate such as a sapphire substrate and a
silicon carbide substrate through a buffer layer, and partitioned by a
plurality of crossing streets to define a plurality of regions where
optical devices such as light emitting diodes and laser diodes are
respectively formed.

[0003] 2. Description of the Related Art

[0004] In an optical device fabrication process, an optical device layer
composed of an n-type semiconductor layer and a p-type semiconductor
layer is formed on the front side of a substantially disk-shaped epitaxy
substrate such as a sapphire substrate and a silicon carbide substrate
through a buffer layer, and this optical device layer is partitioned by a
plurality of crossing streets into a plurality of regions where a
plurality of optical devices such as light emitting diodes and laser
diodes are respectively formed, thus constituting an optical device
wafer. The optical device wafer is divided along the streets to thereby
obtain the individual optical devices (see Japanese Patent Laid-open No.
Hei 10-305420, for example).

[0005] Further, as a technique for improving the luminance of an optical
device, a manufacturing method called lift-off is disclosed in
JP-T-2005-516415. In an optical device wafer, an optical device layer
composed of an n-type semiconductor layer and a p-type semiconductor
layer is formed on the front side of an epitaxy substrate such as a
sapphire substrate and a silicon carbide substrate through a buffer
layer. The above-mentioned method called lift-off includes the steps of
bonding the optical device layer of the optical device wafer through a
bonding metal layer formed of gold (Au), platinum (Pt), chromium (Cr),
indium (In), or palladium (Pd) to a transfer substrate formed of
molybdenum (Mo), copper (Cu), or silicon (Si) and next applying a laser
beam from the back side of the epitaxy substrate to the buffer layer to
thereby peel off the epitaxy substrate, thus transferring the optical
device layer to the transfer substrate.

SUMMARY OF THE INVENTION

[0006] In the technique disclosed in JP-T-2005-516415, the step of bonding
the transfer substrate to the optical device layer formed on the front
side of the epitaxy substrate is performed by heating to 220 to
300° C. Accordingly, warpage occurs in the unit of the epitaxy
substrate and the transfer substrate bonded together because of a
difference in coefficient of linear expansion between the epitaxy
substrate and the transfer substrate. As a result, in peeling off the
epitaxy substrate from the optical device layer, it is difficult to
accurately set the focal point of the laser beam in the buffer layer
formed between the epitaxy substrate and the optical device layer,
causing such a problem that the optical device layer may be damaged or
the buffer layer may not be surely decomposed to result in unsmooth
peeling of the epitaxy substrate.

[0007] It is therefore an object of the present invention to provide an
optical device wafer processing method which can smoothly transfer the
optical device layer formed on the front side of the epitaxy substrate
constituting the optical device wafer to the transfer substrate without
damage to the optical device layer.

[0008] In accordance with an aspect of the present invention, there is
provided an optical device wafer processing method for transferring an
optical device layer in an optical device wafer to a transfer substrate,
the optical device layer being formed on the front side of an epitaxy
substrate through a buffer layer, and partitioned by a plurality of
crossing streets to define a plurality of regions where optical devices
are respectively formed, the optical device wafer processing method
including: a transfer substrate bonding step of bonding the transfer
substrate to the front side of the optical device layer formed on the
front side of the epitaxy substrate through the buffer layer; a transfer
substrate cutting step of cutting the transfer substrate bonded to the
front side of the optical device layer together with the optical device
layer along the streets; a supporting member attaching step of attaching
the transfer substrate to a supporting member after performing the
transfer substrate cutting step; a peeling laser beam applying step of
applying a laser beam having a transmission wavelength to the epitaxy
substrate from the back side of the epitaxy substrate to a unit of the
optical device wafer and the transfer substrate in a condition where the
focal point of the laser beam is set in the buffer layer, thereby
decomposing the buffer layer; and an epitaxy substrate peeling step of
peeling off the epitaxy substrate from the optical device layer after
performing the peeling laser beam applying step.

[0009] Preferably, the transfer substrate cutting step includes the step
of cutting the transfer substrate along the streets by using a cutting
blade. Alternatively, the transfer substrate cutting step includes the
step of cutting the transfer substrate along the streets by applying a
laser beam to the transfer substrate along the streets.

[0010] In the transfer substrate cutting step of the optical device wafer
processing method according to the present invention, the transfer
substrate is cut along the streets to thereby relieve the warpage
occurring in the unit of the epitaxy substrate and the transfer substrate
due to the difference in coefficient of linear expansion between the
epitaxy substrate and the transfer substrate. Accordingly, in performing
the peeling laser beam applying step, the focal point of the laser beam
can be accurately positioned in the buffer layer. The buffer layer is
formed of gallium nitride (GaN), and it is decomposed as
2GaN→2Ga+N2 by the application of the laser beam. Thus, N2 gas is
produced to have an adverse effect on the optical device layer. However,
since the transfer substrate is divided along division grooves into a
plurality of pieces respectively corresponding to the individual optical
devices, the N2 gas produced can be relieved through the division grooves
to thereby reduce the adverse effect on the optical device layer.

[0011] The above and other objects, features and advantages of the present
invention and the manner of realizing them will become more apparent, and
the invention itself will best be understood from a study of the
following description and appended claims with reference to the attached
drawings showing some preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1A is a perspective view of an optical device wafer to be
processed by an optical device wafer processing method according to the
present invention;

[0013] FIG. 1B is an enlarged sectional view of an essential part of the
optical device wafer shown in FIG. 1A;

[0014]FIG. 2A is a perspective view for illustrating a transfer substrate
bonding step in the optical device wafer processing method according to
the present invention;

[0015] FIG. 2B is an enlarged sectional view of an essential part of the
unit of the optical device wafer and the transfer substrate bonded
together by the transfer substrate bonding step shown in FIG. 2A;

[0016]FIG. 3 is a perspective view of an essential part of a cutting
apparatus for performing a first preferred embodiment of a transfer
substrate cutting step in the optical device wafer processing method
according to the present invention;

[0017] FIGS. 4A and 4B are sectional side views for illustrating the first
preferred embodiment of the transfer substrate cutting step in the
optical device wafer processing method according to the present
invention;

[0018]FIG. 4C is an enlarged sectional view of an essential part of the
unit of the optical device wafer and the transfer substrate cut by the
first preferred embodiment of the transfer substrate cutting step shown
in FIGS. 4A and 4B;

[0019]FIG. 4D is a perspective view of the unit of the optical device
wafer and the transfer substrate cut by the first preferred embodiment of
the transfer substrate cutting step shown in FIGS. 4A and 4B;

[0020]FIG. 5 is a perspective view of an essential part of a laser
processing apparatus for performing a second preferred embodiment of the
transfer substrate cutting step in the optical device wafer processing
method according to the present invention;

[0021] FIGS. 6A and 6B are sectional side views for illustrating the
second preferred embodiment of the transfer substrate cutting step in the
optical device wafer processing method according to the present
invention;

[0022]FIG. 6C is an enlarged sectional view of an essential part of the
unit of the optical device wafer and the transfer substrate cut by the
second preferred embodiment of the transfer substrate cutting step shown
in FIGS. 6A and 6B;

[0023]FIG. 6D is a perspective view of the unit of the optical device
wafer and the transfer substrate cut by the second preferred embodiment
of the transfer substrate cutting step shown in FIGS. 6A and 6B;

[0024]FIG. 7 is a perspective view for illustrating an optical device
wafer supporting step in the optical device wafer processing method
according to the present invention;

[0025]FIG. 8 is a perspective view of an essential part of a laser
processing apparatus for performing a peeling laser beam applying step in
the optical device wafer processing method according to the present
invention;

[0026] FIGS. 9A to 9C are views for illustrating the peeling laser beam
applying step in the optical device wafer processing method according to
the present invention; and

[0027] FIG. 10 is a perspective view for illustrating an epitaxy substrate
peeling step in the optical device wafer processing method according to
the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0028] A preferred embodiment of the optical device wafer processing
method according to the present invention will now be described in detail
with reference to the attached drawings. FIG. 1A is a perspective view of
an optical device wafer 2 to be processed by the optical device wafer
processing method according to the present invention, and FIG. 1B is an
enlarged sectional view of an essential part of the optical device wafer
2 shown in FIG. 1A. The optical device wafer 2 shown in FIGS. 1A and 1B
is formed by epitaxial growth of an optical device layer 21 on the front
side 20a of a substantially circular epitaxy substrate 20 such as a
sapphire substrate and a silicon carbide substrate. The optical device
layer 21 is composed of an n-type gallium nitride semiconductor layer 211
and a p-type gallium nitride semiconductor layer 212. In forming the
optical device layer 21 composed of the n-type gallium nitride
semiconductor layer 211 and the p-type gallium nitride semiconductor
layer 212 on the front side 20a of the epitaxy substrate 20 by epitaxial
growth, a buffer layer 22 of gallium nitride (GaN) is formed between the
front side 20a of the epitaxy substrate 20 and the n-type gallium nitride
semiconductor layer 211 forming the optical device layer 21. In the thus
configured optical device wafer 2, the epitaxy substrate 20 has a
thickness of 430 μm, for example, and the optical device layer 21 has
a thickness of 5 μm, for example, inclusive of the thickness of the
buffer layer 22 in the embodiment shown in the figure. As shown in FIG.
1A, the optical device layer 21 is partitioned by a plurality of crossing
streets 23 to define a plurality of regions where a plurality of optical
devices 24 are respectively formed.

[0029] To peel off the epitaxy substrate 20 from the optical device layer
21 in the optical device wafer 2 and transfer the optical device layer 21
to a transfer substrate, a transfer substrate bonding step is first
performed in such a manner that the transfer substrate is bonded to the
front side 21a of the optical device layer 21. More specifically, as
shown in FIGS. 2A and 2B, a transfer substrate 3 having a thickness of
220 μ, for example, is bonded through a bonding metal layer 4 to the
front side 21a of the optical device layer 21 formed on the front side
20a of the epitaxy substrate 20 constituting the optical device wafer 2.
The transfer substrate 3 is formed of molybdenum (Mo), copper (Cu), or
silicon (Si), for example. The bonding metal layer 4 is formed of gold
(Au), platinum (Pt), chromium (Cr), indium (In), or palladium (Pd), for
example, as a bonding metal.

[0030] The transfer substrate bonding step is performed in the following
manner. The bonding metal mentioned above is deposited by evaporation on
the front side 21a of the optical device layer 21 formed on the front
side 20a of the epitaxy substrate 20 or on the front side 3a of the
transfer substrate 3, thereby forming the bonding metal layer 4 having a
thickness of about 3 μm. Thereafter, the bonding metal layer 4 is
brought into pressure contact with the front side 3a of the transfer
substrate 3 or the front side 21a of the optical device 21, thereby
bonding the front side 3a of the transfer substrate 3 through the bonding
metal layer 4 to the front side 21a of the optical device layer 21
constituting the optical device wafer 2. In bonding the transfer
substrate 3 to the front side 21a of the optical device layer 21 formed
on the front side 20a of the epitaxy substrate 20, the epitaxy substrate
20 and the transfer substrate 3 are heated to 220 to 300° C.
Accordingly, warpage occurs in the unit of the epitaxy substrate 20 and
the transfer substrate 3 bonded together because of a difference in
coefficient of linear expansion between the epitaxy substrate 20 and the
transfer substrate 3. In the case that the diameter of the epitaxy
substrate 20 is 10 cm, the amount of this warpage is about 0.5 mm.

[0031] After performing the transfer substrate bonding step, a transfer
substrate cutting step is performed in such a manner that the transfer
substrate 3 is cut together with the optical device layer 21 along the
streets 23. A first preferred embodiment of the transfer substrate
cutting step will now be described with reference to FIG. 3 and FIGS. 4A
to 4D. The first preferred embodiment of the transfer substrate cutting
step is performed by using a cutting apparatus 5 shown in FIG. 3. The
cutting apparatus 5 shown in FIG. 3 includes a chuck table 51 for holding
a workpiece, cutting means 52 having a cutting blade 521 for cutting the
workpiece held on the chuck table 51, and imaging means 53 for imaging
the workpiece held on the chuck table 51. The chuck table 51 is so
configured as to hold the workpiece under suction. The chuck table 51 is
movable in a feeding direction shown by an arrow X in FIG. 3 by feeding
means (not shown) and also movable in an indexing direction shown by an
arrow Y in FIG. 3 by indexing means (not shown). The cutting blade 521 of
the cutting means 52 is rotatable about an axis extending in the indexing
direction and movable in a direction perpendicular to the upper surface
of the chuck table 51. In the preferred embodiment shown in the figure,
the imaging means 53 includes an ordinary imaging device (CCD) for
imaging the workpiece by using visible light, infrared light applying
means for applying infrared light to the workpiece, an optical system for
capturing the infrared light applied to the workpiece by the infrared
light applying means, and an imaging device (infrared CCD) for outputting
an electrical signal corresponding to the infrared light captured by the
optical system. An image signal output from the imaging means 53 is
transmitted to control means (not shown).

[0032] The transfer substrate cutting step using the thus configured
cutting apparatus 5 is performed in the following manner. The unit of the
optical device wafer 2 and the transfer substrate 3 bonded together in
the transfer substrate bonding step mentioned above is placed on the
chuck table 51 of the cutting apparatus 5 in the condition where the
epitaxy substrate 20 of the optical device wafer 2 comes into contact
with the upper surface of the chuck table 51 as shown in FIG. 3.
Accordingly, the back side 3b of the transfer substrate 3 bonded to the
front side 21a of the optical device layer 21 formed on the front side
20a of the epitaxy substrate 20 constituting the optical device wafer 2
is oriented upward. By operating suction means (not shown), the unit of
the optical device wafer 2 and the transfer substrate 3 is held under
suction on the chuck table 51. The chuck table 51 thus holding the unit
of the optical device wafer 2 and the transfer substrate 3 under suction
is moved to a position directly below the imaging means 53 by the feeding
means (not shown).

[0033] When the chuck table 51 is positioned directly below the imaging
means 53, an alignment operation is performed by the imaging means 53 and
the control means (not shown) to detect a cutting area of the transfer
substrate 3. In the case that the transfer substrate 3 is formed from a
silicon substrate, the imaging means 53 and the control means (not shown)
perform image processing such as pattern matching for making the
alignment between the cutting blade 521 and the streets 23 extending in a
first direction on the optical device layer 21 of the optical device
wafer 2, thereby performing the alignment in the cutting area for the
streets 23 extending in the first direction (alignment step). Similarly,
the imaging means 53 and the control means perform the alignment in the
cutting area for the other streets 23 extending in a second direction
perpendicular to the first direction on the optical device layer 21 of
the optical device wafer 2.

[0034] Although the transfer substrate 3 is present on the upper side of
the optical device layer 21 where the streets 23 are formed, the streets
23 can be imaged through the transfer substrate 3 formed from a silicon
substrate because the imaging means 53 includes the infrared light
applying means for applying infrared light, the optical system for
capturing the infrared light, and the imaging device (infrared CCD) for
outputting an electrical signal corresponding to the infrared light as
mentioned above. In the case that the transfer substrate 3 is formed from
a metal substrate, a holding portion of the chuck table 51 for holding
the workpiece is formed from a transparent member and the streets 23 are
imaged from the lower side of the holding portion.

[0035] After performing the alignment operation for detecting the cutting
area of the transfer substrate 3 bonded to the optical device wafer 2
held on the chuck table 51, the chuck table 51 holding the transfer
substrate 3 bonded to the optical device wafer 2 is moved to a cutting
start position below the cutting blade 521 as shown in FIG. 4A. At this
cutting start position, one end (left end as viewed in FIG. 4A) of a
predetermined one of the streets 23 extending in the first direction is
positioned on the right side of the cutting blade 521 by a predetermined
small amount as shown in FIG. 4A. Thereafter, the cutting blade 521 is
rotated in the direction shown by an arrow 521a in FIG. 4A and
simultaneously moved down by a predetermined amount in the direction
shown by an arrow Z1 in FIG. 4A from a standby position shown by a
two-dot chain line in FIG. 4A to a working position shown by a solid line
in FIG. 4A. This movement from the standby position to the working
position is performed by operating vertically moving means (not shown)
for moving the cutting blade 521 in the direction perpendicular to the
upper surface of the chuck table 51. The working position of the cutting
blade 521 is set so that the outer circumference of the cutting blade 521
reaches the buffer layer 22. After moving the cutting blade 521 from the
standby position to the working position as mentioned above, the chuck
table 51 is moved at a predetermined feed speed in the direction shown by
an arrow X1 in FIG. 4A as rotating the cutting blade 521 in the direction
shown by the arrow 521a. When the other end (right end as viewed in FIG.
4B) of the predetermined street 23 extending in the first direction
reaches a position on the left side of the cutting blade 521 by a
predetermined small amount as shown in FIG. 4B, the movement of the chuck
table 51 is stopped. Thereafter, the cutting blade 521 is raised to the
standby position shown by a two-dot chain line in FIG. 4B in the
direction shown by an arrow Z2 in FIG. 4B.

[0036] As a result, the transfer substrate 3 and the optical device layer
21 formed on the front side of the epitaxy substrate 20 constituting the
optical device wafer 2 are cut along the predetermined street 23
extending in the first direction to form a division groove 31 as a cut
groove as shown in FIG. 4C (transfer substrate cutting step). The
transfer substrate cutting step is performed along all of the crossing
streets 23 extending in the first direction and the second direction
perpendicular to the first direction to thereby form a plurality of
crossing division grooves 31 on the transfer substrate 3 along all of the
crossing streets 23 as shown in FIG. 4D. As mentioned above, the unit of
the epitaxy substrate 20 and the transfer substrate 3 has a warpage of
about 0.5 mm. This warpage is relieved to some extent by the suction on
the chuck table 51, but does not become zero. Accordingly, there is a
case that the epitaxy substrate 20 may be cut by the cutting blade 521.

[0037] A second preferred embodiment of the transfer substrate cutting
step will now be described with reference to FIG. 5 and FIGS. 6A to 6D.
The second preferred embodiment of the transfer substrate cutting step is
performed by using a laser processing apparatus 6 shown in FIG. 5. The
laser processing apparatus 6 shown in FIG. 5 includes a chuck table 61
for holding a workpiece, laser beam applying means 62 for applying a
laser beam to the workpiece held on the chuck table 61, and imaging means
63 for imaging the workpiece held on the chuck table 61. The chuck table
61 is so configured as to hold the workpiece under suction. The chuck
table 61 is movable in a feeding direction shown by an arrow X in FIG. 5
by feeding means (not shown) and also movable in an indexing direction
shown by an arrow Y in FIG. 5 by indexing means (not shown).

[0038] The laser beam applying means 62 includes a cylindrical casing 621
extending in a substantially horizontal direction and focusing means 622
mounted on the front end of the casing 621 for applying a pulsed laser
beam to the workpiece held on the chuck table 61. The imaging means 63 is
mounted on the front end portion of the casing 621 constituting the laser
beam applying means 62. In the preferred embodiment shown in the figure,
the imaging means 63 includes an ordinary imaging device (CCD) for
imaging the workpiece by using visible light, infrared light applying
means for applying infrared light to the workpiece, an optical system for
capturing the infrared light applied to the workpiece by the infrared
light applying means, and an imaging device (infrared CCD) for outputting
an electrical signal corresponding to the infrared light captured by the
optical system. An image signal output from the imaging means 63 is
transmitted to control means (not shown).

[0039] The transfer substrate cutting step using the laser processing
apparatus 6 will be described with reference to FIG. 5 and FIGS. 6A to
6D. The transfer substrate cutting step using the laser processing
apparatus 6 is performed in the following manner. The unit of the optical
device wafer 2 and the transfer substrate 3 bonded together in the
transfer substrate bonding step mentioned above is placed on the chuck
table 61 of the laser processing apparatus 6 in the condition where the
epitaxy substrate 20 of the optical device wafer 2 comes into contact
with the upper surface of the chuck table 61 as shown in FIG. 5.
Accordingly, the back side 3b of the transfer substrate 3 bonded to the
front side 21a of the optical device layer 21 formed on the front side
20a of the epitaxy substrate 20 constituting the optical device wafer 2
is oriented upward. By operating suction means (not shown), the unit of
the optical device wafer 2 and the transfer substrate 3 is held under
suction on the chuck table 61. The chuck table 61 thus holding the unit
of the optical device wafer 2 and the transfer substrate 3 under suction
is moved to a position directly below the imaging means 63 by the feeding
means (not shown).

[0040] When the chuck table 61 is positioned directly below the imaging
means 63, an alignment operation is performed by the imaging means 63 and
the control means (not shown) to detect a processing area of the transfer
substrate 3 to be laser-processed. In the case that the transfer
substrate 3 is formed from a silicon substrate, the imaging means 63 and
the control means (not shown) perform image processing such as pattern
matching for making the alignment between the streets 23 extending in the
first direction and the focusing means 622 of the laser beam applying
means 62 for applying a laser beam along the streets 23 on the optical
device layer 21 of the optical device wafer 2, thereby performing the
alignment in the processing area for the streets 23 extending in the
first direction (alignment step). Similarly, the imaging means 63 and the
control means perform the alignment in the processing area for the other
streets 23 extending in the second direction perpendicular to the first
direction on the optical device layer 21 of the optical device wafer 2.

[0041] Although the transfer substrate 3 is present on the upper side of
the optical device layer 21 where the streets 23 are formed, the streets
23 can be imaged through the transfer substrate 3 formed from a silicon
substrate because the imaging means 63 includes the infrared light
applying means for applying infrared light, the optical system for
capturing the infrared light, and the imaging means (infrared CCD) for
outputting an electrical signal corresponding to the infrared light as
mentioned above. In the case that the transfer substrate 3 is formed from
a metal substrate, a holding portion of the chuck table 61 for holding
the workpiece is formed from a transparent member and the streets 23 are
imaged from the lower side of the holding portion.

[0042] After performing the alignment operation for detecting the
processing area of the transfer substrate 3 bonded to the optical device
wafer 2 held on the chuck table 61, the chuck table 61 holding the
transfer substrate 3 bonded to the optical device wafer 2 is moved to a
processing start position below the focusing means 622 of the laser beam
applying means 62 as shown in FIG. 6A. At this processing start position,
one end (left end as viewed in FIG. 6A) of a predetermined one of the
streets 23 extending in the first direction is positioned directly below
the focusing means 622 of the laser beam applying means 62 as shown in
FIG. 6A. Thereafter, a pulsed laser beam having an absorption wavelength
to the transfer substrate 3 is applied from the focusing means 622 to the
transfer substrate 3, and the chuck table 61 is moved in the direction
shown by an arrow X1 in FIG. 6A at a predetermined feed speed. When the
other end (right end as viewed in FIG. 6B) of the predetermined street 23
reaches the position directly below the focusing means 622 of the laser
beam applying means 62 as shown in FIG. 6B, the application of the pulsed
laser beam is stopped and the movement of the chuck table 61 is also
stopped (laser beam applying step). In this laser beam applying step, the
focal point P of the pulsed laser beam is set near the back side 3b
(upper surface) of the transfer substrate 3. The above-mentioned laser
beam applying step is performed along all of the streets 23 formed on the
optical device layer 21 of the optical device wafer 2.

[0043] For example, the laser beam applying step mentioned above is
performed under the following processing conditions.

[0044] Light source: YAG pulsed laser

[0045] Wavelength: 355 nm

[0046] Average power: 7 W

[0047] Repetition frequency: 10 kHz

[0048] Focused spot diameter: ellipse having a minor axis set to 10 μm
and a major axis set to 10 to 200 μm

[0049] Work feed speed: 100 mm/s

[0050] The laser beam applying step is repeated 4 to 6 times along each
street 23 under the above processing conditions. As a result, the
transfer substrate 3 and the optical device layer 21 formed on the front
side of the epitaxy substrate 20 constituting the optical device wafer 2
are cut along the predetermined street 23 extending in the first
direction to form a division groove 31 as a laser processed groove as
shown in FIG. 6C (transfer substrate cutting step). The transfer
substrate cutting step is performed along all of the crossing streets 23
extending in the first direction and the second direction perpendicular
to the first direction to thereby form a plurality of crossing division
grooves 31 on the transfer substrate 3 along all of the crossing streets
23 as shown in FIG. 6D.

[0051] By performing the transfer substrate cutting step to cut the
transfer substrate 3 and the optical device layer 21 along all of the
crossing streets 23 as described above, it is possible to relieve the
warpage occurring in the unit of the epitaxy substrate 20 and the
transfer substrate 3 due to the difference in coefficient of linear
expansion between the epitaxy substrate 20 and the transfer substrate 3.

[0052] After performing the transfer substrate cutting step along all of
the crossing streets 23, an optical device wafer supporting step is
performed in such a manner that the unit of the optical device wafer 2
and the transfer substrate 3 is attached to a dicing tape as a supporting
member supported to an annular frame. More specifically, as shown in FIG.
7, the transfer substrate 3 bonded to the optical device wafer 2 is
attached to the front side (upper surface) of a dicing tape T as a
supporting member supported to an annular frame F (supporting member
attaching step). Accordingly, the back side 20b of the epitaxy substrate
20 of the optical device wafer 2 bonded to the transfer substrate 3
attached to the upper surface of the dicing tape T is oriented upward.

[0053] After performing the optical device wafer supporting step as
described above, a peeling laser beam applying step is performed in such
a manner that a laser beam having a transmission wavelength to the
epitaxy substrate 20 is applied from the back side 20b of the epitaxy
substrate 20 to the optical device wafer 2 in the condition where the
focal point of the laser beam is set in the buffer layer 22 to decompose
the buffer layer 22. This peeling laser beam applying step is performed
by using a laser processing apparatus 7 shown in FIG. 8. The laser
processing apparatus 7 shown in FIG. 8 includes a chuck table 71 for
holding a workpiece and laser beam applying means 72 for applying a laser
beam to the workpiece held on the chuck table 71. The chuck table 71 is
so configured as to hold the workpiece under suction. The chuck table 71
is movable in a feeding direction shown by an arrow X in FIG. 8 by
feeding means (not shown) and also movable in an indexing direction shown
by an arrow Y in FIG. 8 by indexing means (not shown). The laser beam
applying means 72 includes a cylindrical casing 721 extending in a
substantially horizontal direction and focusing means 722 mounted on the
front end of the casing 721 for applying a pulsed laser beam to the
workpiece held on the chuck table 71.

[0054] The peeling laser beam applying step using the laser processing
apparatus 7 will now be described with reference to FIG. 8 and FIGS. 9A
to 9C. As shown in FIG. 8 described above, in the peeling laser beam
applying step, the unit of the optical device wafer 2, the transfer
substrate 3, and the dicing tape T bonded together is placed on the chuck
table 71 of the laser processing apparatus 7 in the condition where the
dicing tape T comes into contact with the upper surface of the chuck
table 71. By operating suction means (not shown), the unit of the optical
device wafer 2, the transfer substrate 3, and the dicing tape T is held
under suction on the chuck table 71. Accordingly, the back side 20b of
the epitaxy substrate 20 of the optical device wafer 2 bonded to the
transfer substrate 3 attached to the dicing tape T held on the chuck
table 71 is oriented upward. While the annular frame F supporting the
dicing tape T is not shown in FIG. 8, the annular frame F is held by any
suitable frame holding means provided on the chuck table 71.

[0055] Thereafter, the chuck table 71 holding the unit of the optical
device wafer 2, the transfer substrate 3, and the dicing tape T under
suction is moved to a processing start position below the focusing means
722 of the laser beam applying means 72 as shown in FIG. 9A. At this
processing start position, one end (left end as viewed in FIG. 9A) of the
epitaxy substrate 20 is positioned directly below the focusing means 722
of the laser beam applying means 72 as shown in FIG. 9A. Thereafter, the
focal point P of the pulsed laser beam to be applied from the focusing
means 722 is set in the buffer layer 22 as shown in FIG. 9B. Then, the
laser beam applying means 72 is operated to apply the pulsed laser beam
from the focusing means 722 to the buffer layer 22, and the chuck table
71 is moved in the direction shown by an arrow X1 in FIG. 9A at a
predetermined feed speed. When the other end (right end as viewed in FIG.
9C) of the epitaxy substrate 20 reaches the position directly below the
focusing means 722 of the laser beam applying means 72 as shown in FIG.
9C, the application of the pulsed laser beam is stopped and the movement
of the chuck table 71 is also stopped (peeling laser beam applying step).
This peeling laser beam applying step is performed over the entire
surface of the buffer layer 22. As a result, the buffer layer 22 is
decomposed to lose its binding function of binding the epitaxy substrate
20 and the optical device layer 21.

[0056] For example, the peeling laser beam applying step mentioned above
is performed under the following processing conditions.

[0057] Light source: excimer pulsed laser

[0058] Wavelength: 284 nm

[0059] Average power: 0.08 W

[0060] Repetition frequency: 50 kHz

[0061] Focused spot diameter: φ 400 μm

[0062] Work feed speed: 20 mm/s

[0063] In the transfer substrate cutting step mentioned above, the
transfer substrate 3 is cut along the crossing streets 23 to thereby
relieve the warpage occurring in the unit of the epitaxy substrate 20 and
the transfer substrate 3 due to the difference in coefficient of linear
expansion between the epitaxy substrate 20 and the transfer substrate 3.
Accordingly, in performing the peeling laser beam applying step, the
focal point P of the pulsed laser beam to be applied from the focusing
means 722 can be accurately positioned in the buffer layer 22. The buffer
layer 22 is formed of gallium nitride (GaN), and it is decomposed as
2GaN→2Ga+N2 by the application of the laser beam. Thus, N2 gas is
produced to have an adverse effect on the optical device layer 21.
However, since the transfer substrate 3 is divided along the division
grooves 31 into a plurality of pieces respectively corresponding to the
individual optical devices 24 in the transfer substrate cutting step, the
N2 gas produced can be relieved through the division grooves 31 to
thereby reduce the adverse effect on the optical device layer 21.

[0064] After performing the peeling laser beam applying step described
above, an epitaxy substrate peeling step is performed in such a manner
that the epitaxy substrate 20 is peeled off from the optical device layer
21. More specifically, by performing the peeling laser beam applying
step, the binding function of the buffer layer 22 binding the epitaxy
substrate 20 and the optical device layer 21 is lost. Accordingly, the
epitaxy substrate 20 can be easily peeled off from the optical device
layer 21 as shown in FIG. 10. By performing the transfer substrate
cutting step, the transfer substrate 3 and the optical device layer 21
are divided along the division grooves 31 into a plurality of pieces 30
respectively corresponding to the individual optical devices 24.
Accordingly, by performing the epitaxy substrate peeling step, the plural
pieces 30 divided from the transfer substrate 3 attached to the dicing
tape T supported to the annular frame F are obtained in the condition
that the individual optical devices 24 are respectively bonded to the
individual pieces 30 of the transfer substrate 3.

[0065] The present invention is not limited to the details of the above
described preferred embodiments. The scope of the invention is defined by
the appended claims and all changes and modifications as fall within the
equivalence of the scope of the claims are therefore to be embraced by
the invention.